For some reason I am thinking you want to match it up with the 1100 Lampredi engine.
I used to own an 86 (or was it 88 1100 with 5 speed), it was a top loader box, however, some bottom loader boxes will fit. I have just fitted one to my 84 70s (1300).
I have the original box out of my 1300 which I am sure would fit (if you lived here) however it has a chip off a cluster gear. Not sure how it happened, but it happened while I was driving. Car has 270,000km on the odo, so gearbox has gone a fair way.
Any way, this is all to say they can be obtained.
the later bottom loader (for the gearshift) are a little different.
Bigger inner driveshaft tripod bearings, that can be fitted to the smaller driveshafts.
And different gearshift set up which you need to swap over.
I have just done it to mine with the above gearbox change.
The clutch is also different, larger splines, on a larger input shaft.
